This invention relates to an exhaust gas recirculation control system for returning part of the exhaust gas of an internal combustion engine to the intake manifold thereof, or more in particular to a self-diagnosis system for the exhaust gas recirculation control system.
Conventionally, exhaust gas recirculation control systems of this type (hereinafter referred to as "EGR") find wide applications as means of reducing nitrogen oxides (NOX).
In the case where an operating failure of an EGR valve or EGR pipe clogged causes an EGR fault, NOX is likely to increase extremely. Such an EGR fault, which little affects the operating performance, however, may result in an increased amount of NOX emitted or the pollution of the atmosphere without the knowledge of the driver.
A means to overcome this problem is well known, in which the detected value from a sensor is corrected by the learning control or the like according to a predetermined pattern, such a fault is announced when the correction value exceeds a predetermined value.
Application of the above-mentioned prior art to EGR, however, requires a flow rate sensor or the like to be installed on the EGR pipe for detecting the EGR operating conditions, thereby posing the problem of a complicated construction.